There are a large number of patents relating to blends of polyolefins and particularly blends of low density polyethylene and a high density polyethylene.
U.S. Pat. No. 5,306,775 issued Apr. 26, 1994 to Martin et al., assigned to Phillips Petroleum Company discloses a blend of comprising from 70 to 30 weight % of a low molecular weight ethylene homopolymer having a density greater than 0.960 g/cm3 and melt index (2.16 kg, 190° C.) greater than 30 gm per 10 minutes made in the presence of a chromium catalyst and from 30 to 70 weight % of a high molecular weight ethylene copolymer having a density less than 0.955 g/cm3 and a high load melt index (21.6 kg 190° C.) from 0.1 to 50 g/10 minutes made in the presence of a titanium chloride based catalyst system. The reference teaches away from the present invention in that the chrome catalyst component is a homopolymer having a higher density and a significantly higher melt index than those of the present invention.
WO 2001/005852 filed Jul. 15, 1999 in the name of Van Dun, Jozef et al., assigned to Dow Global Technologies Inc. discloses a polymer blend composition comprising; (A) an ethylene homopolymer in an amount of from 1 to 99 percent by weight based on the combined weight of components (A) and (B), having a ratio of Mw/Mn (polydispersity) (as measured by GPC) greater than about 2.5, a density which satisfies the following inequality; density (g/cm3)>0.9611+0.0058 log I2−0.00128] log2 I2; and an Mw/Mn ratio which satisfies the following inequality; Mw/Mn≦11.67 log [Mw−43.67;] and (B) an ethylene homopolymer other than one of component A having the identical properties or an ethylene alpha olefin copolymer in an amount of from 1 to 99 percent by weight based on the combined weight of components (A) and (B). The reference teaches away from the present invention in that two homopolymers are not present and no component in the present invention has a density greater than 0.960 g/cm3.
U.S. Pat. No. 6,350,814 issued Feb. 26, 2002 to Bauer et al., assigned to Basell Polyolefine GmbH teaches a blend of a polyethylene made using a metallocene catalyst and a polyethylene made using a Ziegler Natta or Phillips catalyst. The polyethylene made using the metallocene catalyst has a density from 0.91 to 0.93 g cm3 and the polyethylene made using the Ziegler Natta or Phillips catalyst has a density from 0.930 to 0.945 g/cm3. There is no component in the blend having a density greater than 0.953 g/cm3.
U.S. Pat. No. 6,245,867 issued Jun. 12, 2001 to Debras, assigned to Fina Research S.A. teaches a process to prepare a blend of polyethylenes made using tandem or sequential reactors. An ethylene homopolymer is made using the chrome based catalyst and an ethylene homopolymer or copolymer is made in the second reactor using either a Ziegler Natta catalyst or a metallocene catalyst. The polymers are not broadly described in the specification. However, the examples give some of the properties of the different components. The ethylene homopolymer made with the chrome based catalyst has a density from 0.964 g/cc this is greater than the density for the ethylene polymer of the present invention made using a chrome catalyst. The density of the ethylene copolymer made using the Ziegler Natta catalyst is above 0.940 g/cc which is greater than the density for the ethylene copolymer of the present invention made using a metallocene catalyst. The reference teaches away from the subject matter of the present invention.
Canadian Patent application 2,078,309 laid open Mar. 19, 1993 in the name of Martin et al., assigned to Phillips Petroleum Company teaches a polyethylene blend comprising less than or equal to 35 weight % of a high molecular weight polyethylene prepared in the presence of a Ziegler Natta titanium based catalyst having a HLMI from 0.2 to 25 g/10 minutes, a density from 0.910 to 0.940 g/cc and a polydispersity less than about 10 and greater than 65 weight % of a low molecular weight polyethylene prepared in the presence of a chrome based catalyst having a density from 0.945 g/cc to 0.980 g/cc, a melt index from 0.1 to 5 g/10 minutes and a polydispersity greater than or equal to about 6.
The present patent application seeks to provide a blend of a higher density low molecular weight polyethylene prepared in the presence of the chromium or a Ziegler Natta type catalyst and a lower density higher molecular weight polyethylene prepared in the presence of a single site type catalyst.